Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors (OFETs) (see Katz, Bao and Gilat, Acc. Chem. Res., 2001, 34, 5, 359). Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
In prior art regioregular head-to-tail (HT) poly-(3-alkylthiophene) (P3AT), in particular poly-(3-hexylthiophene) (P3HT), has been suggested for use as semiconducting material, as it shows charge carrier mobility between 1×10−5 and 0.1 cm2 V−1 s−1. P3AT is a semi-conducting polymer that has shown good performance as the active hole transporting layer in field effect transistors (see Sirringhaus et al, Nature, 1999, 401, 685-688), and photovoltaic cells (see Coakley, McGehee et al., Chem. Mater., 2004, 16, 4533). The charge carrier mobility, and hence the performance of these applications, have been shown to be strongly dependent on the regiorepositioning (or regioregularity) of the alkyl sidechains of the polymer backbone. A high regioregularity means a high degree of head-to-tail (HT) couplings and a low amount of head-to-head (HH) couplings or tail-to-tail (TT) couplings as shown below:

This leads to good packing of the polymers in the solid state and high charge carrier mobility.
Typically a regioregularity greater than 90% is necessary for good performance. In addition to high regioregularity, high molecular weights are desirable in order to enhance the processability and printability of formulations of P3AT. Higher molecular weights also result in increased glass transition temperatures for the polymer, whereas low glass transition temperatures can cause device failure during operation because of unwanted morphological changes occurring at raised temperatures.
Several methods to produce highly regioregular HT-P3AT have been reported in prior art, for example in the review of R. D. McCullough, Adv. Mater., 1998, 10(2), 93-116 and the references cited therein.
For example, regioregular polymers have been prepared by the “Stille-method” (see Stille, Iraqi, Barker et al., J. Mater. Chem., 1998, 8, 25) as illustrated below
or by the “Suzuki-method” (see Suzuki, Guillerez, Bidan et al., Synth. Met., 1998, 93, 123) as shown below.

However, both of these methods have the drawback of requiring an additional process step to obtain and purify the organometallic intermediate.
Other known methods to prepare HT-P3AT with a regioregularity≧90%, starting from 2,5-dibromo-3-alkylthiophene, include for example the “Rieke method”, wherein the educt (wherein R is alkyl) is reacted with highly reactive zinc in THF as illustrated below and disclosed e.g. in WO 93/15086 (A1).

The resulting organozinc species is then reacted with a nickel (II) catalyst, Ni(dppe)Cl2, to afford the polymer. Reaction with a nickel (0) catalyst, Ni(PPh3)4, was reported to afford a polymer of lower regioregularity (65%). Reaction with a palladium (0) catalyst (Pd(PPh3)4) was also reported to afford a polymer of low regioregularity (50%) (see Chen, J. Am. Chem. Soc., 1992, 114, 10087).
Also known is the method to prepare regioregular HT-P3AT as described in McCullough et al., Adv. Mater., 1999, 11(3), 250-253 and in EP 1 028 136 A1 and U.S. Pat. No. 6,166,172. According to this route the educt is reacted with methylmagnesium bromide in THF as shown below.

The resulting organomagnesium reagent is reacted with a nickel (II) catalyst to afford the regioregular polymer. In McCullough et al., Macromolecules, 2005, 38, 8649, this reaction is further investigated. This reference reports that the nickel (II) acts as an initiator in a “living” type polymerization, that the molecular weight of the polymer is related to the concentration of nickel (II) catalyst, and that number average molecular weights (Mn) in the region of 10,000 with polydispersities around 1.5 are obtained.
Both the Rieke and McCullough methods specify the use of a nickel (II) catalyst in order to obtain polymer of high regioregularity. Molecular weights (Mn) in the region of 20-35,000 were reported.
Yamamoto, T. Macromolecules, 1992, 25, 1214 reports the polymerisation of 2,5-dibromo-3-alkylthiophene by adding a stoichiometric amount of bis(1,5-cyclooctadiene)nickel in the presence of a monodentate phosphine ligand as shown below.

However, this method only afforded polymer of low regioregularity (65%) and intermediate molecular weight (Mn=15,000). Besides, the use of stoichiometric amounts of Ni(COD)2 is highly undesirable due to the toxicity of this reagent.
WO 2005/014691 A1 discloses a process for preparing regioregular HT-P3AT by reacting a 2,5-dihalo-3-alkylthiophene with pure magnesium to form a regiochemical Grignard intermediate, and polymerising said intermediate by adding a catalyst, for example a Ni(II) catalyst.
For some applications, especially in FETs, P3ATs with molecular weights higher than those reported in prior art are desirable. High molecular weight polymers offer several advantages: As the molecular weight of a polymer increases, most properties scale with molecular weight until a plateau is reached, at which there is typically little further dependence. It is desirable to achieve molecular weights well above this plateau region in order to minimise a variation in performance with molecular weight, and hence minimise batch to batch discrepancies. Due to physical entanglements that occur in polymers of molecular weight above the plateau region, the mechanical properties improve. In addition, printing formulations of high molecular weight polymers can achieve high enough viscosity to be applied in a range of graphical arts printing processes including offset and gravure, whereas the typical viscosity achieved by regular P3HT of less than 10 centipoise would not suffice for such processes.
The non-prepublished international patent application WO2006/010267 discloses a process of preparing regioregular HT-P3AT or HT-poly(3-alkyselenophene) (P3AS) by reacting a 2,5-dihalo-3-alkylthiophene or -selenophene with an appropriate Grignard or zinc reagent, and bringing the resulting organomagnesium or organozinc intermediate in contact with a catalytic amount of a nickel (0) catalyst and a bidentate ligand to start a polymerisation reaction. It reports that the use of a Ni(0) catalyst instead of a Ni(II) catalyst results in a highly reactive catalyst system, and yields polymers of very high molecular weights and high regioregularity.
On the other hand, if the molecular weight of the polymer is too high, its solubility in organic solvents and thereby its processibility in the formation of semiconductor devices can deteriorate. Therefore, the process of preparing the polymer should also allow to control its molecular weight.
Thus, there is still a need for an improved method of preparing polymers, in particular P3AT and P3AS, with high regioregularity, predictable and defined molecular weight, good processibility, high purity and high yields in an economical, effective and environmentally beneficial way, which is especially suitable for industrial large scale production.
It was an aim of the present invention to provide an improved process for preparing polymers having these advantages, but not having the drawbacks of prior art methods mentioned above. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by a process as described below. In particular, it was surprisingly found that it is possible to prepare P3AT or P3AS with high regioregularity and predictable molecular weight by a process similar to that disclosed in WO2006/010267, but wherein the catalyst activation reaction and the polymerisation reaction are separated. This can be achieved by first activating the Ni(0) complex by addition of a small amount of a halothiophene or haloselenophene derivative, and then bringing the activated complex in contact with the organomagnesium or organozinc intermediate that has been created separately as described above. This process allows a reproducible activation of the Ni(0) complex prior to the polymerisation reaction, and enables a better control over the molecular weight of the polymer thereby obtained.